Computer input/output equipment

ABSTRACT

A digitising table unit for use with a digital computer having two parallel table surfaces with a digitising pen on the top table surface and a bug below that surface movable along x and y axes by servo-motors coordinated with the pen, and retractable pen means on the bug to engage the lower table surface to trace out a representation on the lower table surface with encoding of the bug position in signals to the computer.

O United States Patent 1191 1111 3,763,479 Jebb et al. Oct. 2, 1973COMPUTER INPUT/OUTPUT EQUIPMENT 3,519,905 7/1970 Little et al. 235/151.11 [76] Inventors: Alan Jebb, 14 Cheyne Close Keston' aaslschke BSHSLHather 235/l97 Bmvde" Beam, 7 Andrews 3,593,615 7 1971 Shelton 33 23 cWay, Marlow, both of England 3,320,409 5/1967 Larrowe 235/197 3,225,27912/1965 Muldoon et al. 1. 346/29 [22] 1971 3,422,537 1/1969 Dewey etal.33/23 c 21 Appl. No.: 210,763

Primary ExaminerGareth D. Shaw [30] Foreign Application Priority DataA'mmey Kafl mocks ec 22, 1970 Great Brltam 60,860/70 ABSTRACT 52 us. c1.340/1725, 346/29 A digitising table unit for use with a digital computer[51 1 Int. Cl. Gold 9/00 having 1W0 Parallel table Surfaces with adigitising P [58] Field of Search 346/29; 33/] M 18 R on the top tablesurface and a bug below that surface 33/23 C, 24 R; 235 151 1 197; 3401725 movable along x and y axes by servo-motors coordinated with thepen, and retractable pen means on the [56] Ref e Ci d bug to engage thelower table surface to trace out a rep- UNITED STATES PATENTSresentation on the lower table surface with encoding of 3,601,590 8/[971Norton 235/l5l.l l the bug posltlon m slgnals to the computer. 3,473,15710/1969 Little et al, 346/29 23 Claims, 18 Drawing Figures .479 PAIENIED3.763

sum 01 or 13 PATENTED [JET 2 5 saw on or 13 PATENIED OCT 2 Im s m 05 m13 PATENTED 2 I973 sum 07 or 13 sum as or 13 PATENTED 2 mm w PATENTEDGET 2 sum 12 or 13 PATENTEUUET 2197s sum 13 ur 1 COMPUTER INPUT/OUTPUTEQUIPMENT BACKGROUND OF THE INVENTION This invention relates to a fullyinter-active computer aided design system and provides an input/outputdigitising/drawing table for connection on-line" to a computer through asuitable interface.

At the present time, in a variety of fields of application, more andmore use of computers is being attempted for assisting in design orplanning operations. With this in mind, attempts are being made tocommit the contents of engineering drawings, or other data sheets suchas plans of buildings, planning layouts of piping or electrical wiring,or even maps of sites of areas of country, to the memory storage of acomputer, so that the computer may be called upon at a subsequent timeto give a print-out" or draw-out" of that data in the form of a drawingon a co-ordinate drawing table.

Various methods have been proposed for translating the information takenfrom a drawing into a suitable encoded form for assimilation by acomputer, one such method being the use of a digitising table.

Such a digitising table would comprise a surface upon which a drawing orother plan sheet may be placed together with a free probe or digitisingpen which could be used to point to particular items or positions on thedrawing. The table would include means to sense the position of the penat any chosen instant of time and convert that position into x and yco-ordinates with respect to an origin on the surface of the table, andto encode those x and y co-ordinates into electrical signals forsubsequent transmission to the computer.

In previously proposed systems the data was digitised and committed tolong term memories stores in the computer, such as magnetic tape, andwould be used to produce a drawing at a later date with the computercontrolling a separate output table on which a pen" or other markinginstrument was driven across a drawing sheet by suitable x and y andco-ordinate servo-motor systems.

A further proposal which has been used in this field is the provision ofa cathode ray tube display connected to an output ofa computer, andassociated with a "light pen" with which a picture or drawing can be"drawn" on the face of the cathode ray tube leaving behind anilluminated trace. Such display systems have been of two basic types;one using a storage type of cathode ray tube, in which the trace ismaintained on the face of the tube, by physical characteristics of thephosphors, for a period of some hours, and being only erasable by meansof flooding the whole phosphor face with electrons from within the tube;and the other being the video type of display in which the cathode raytube face is continually scanned by an electron beam in the manner of atelevision picture, with the display or picture drawn thereon beingre-created at each frame of the scanning. The first of these systemssuffered from the difficulty of erasing and correcting a drawing in thatthe whole picture had to be re-drawn up to the point of the error eachtime an erasure and correction was required, and both of these systemscan suffer from difficulties in establishing accurate dimensional andcoordinate positions to scale on the drawing. The second of the systemsalso suffers from the considerable disadvantage that it consumes a largecapacity of core store within the computer simply to maintain a pictureof any complexity on the face of the tube as it is scanned. It can beseen that the capacity of core store consumed is proportional to theresolution of the picture and has been typically of the order of 12K to16K bits of store capacity which has been a handicap not only in thecapital cost but in the operation of more sophisticated programmesduring use of the display.

Both these previous systems have also suffered the disadvantage that apermanent or hard copy of a drawing could only be obtainedphotographically from the face of the tube, or an additional plotter.

It is an object of the present invention to alleviate the disadvantagesof the previous systems.

SUMMARY OF THE INVENTION Accordingly, in one aspect, the presentinvention provides an input/output digitising/drawing table unit forconnection on-line to a computer, comprising a first or digitising tablesurface, a freely moveable digitising pen which may be placed at anychosen position within the area of said first table surface, a bugmounted below said first table surface to be moveable along x and y axesby means of servo-motors to a position corresponding to the position ofsaid digitising pen when it is placed on said first table surface,encoding means by which the x and y co-ordinates of the position of saidpen on said first table surface may be converted to electrical signalsfor transmission to a computer, a second table surface occupying a planeparallel to that of said first table surface and a retractable penmounted on said bug whereby movement of said bug may trace out arepresentation on said second table surface under the control of acomputer.

According to a further aspect, the present invention provides a tableunit of the preceding paragraph connected on-line to a digital computerthrough a suitable interface.

According to yet a further aspect, the present invention provides thecombination of the preceding paragraph with the computer programmed toreceive and store co-ordinate information of positions on said firsttable surface selected with said pen, together with instruction datarelating to those points, and subsequently to control said bug and saidpen in movement over said second table surface to produce arepresentation in accordance with that co-ordinate information andinstruction data.

Preferably in the combination of the preceding paragraph, the firsttable surface is provided with a menu card delineating a plurality ofdistinct areas within which said pen may be placed, each said area beingassociated with a particular instruction for the computer, and thecomputer is arranged to locate that part of its programme associatedwith that one of said plurality of said area within which said pen maybe placed.

According to yet a further aspect, the present invention provides in thecombination of either of the preceding a visible display on a cathoderay tube of the contents of stored digitised information.

According to yet a further aspect, the present invention provides aprogramme for the computer in the combination of the last fourparagraphs.

BRIEF DESCRIPTION OF THE DRAWINGS In order to promote a fullerunderstanding of the above, and other, aspects of the present invention,some embodiments will now be described, by way of example only, withreference to the accompanying drawings in which FIG. 1 is a plan view ofan input-output table,

FIG. 2 is a side view in part cross-section of the table of FIG. 1,

FIG. 3 is an end view in part cross-section of the table of FIG. 1,

FIG. 4 is a more detailed side view in part crosssection taken of thepen carrier of the table of FIG. 1 taken on the arrow IV of that Figure,

FIG. 5 is a plan view of the pen index mechanism of FIG. 4,

FIG. 6 is a schematic perspective view of a digitising pen or index foruse with the table of FIG. 1,

FIG. 7 is a plan view of the left hand x-axis drive servo of the tableof FIG. 1,

FIG. 8 is a part cross-sectional view on the arrow VIII of FIG. 7,

FIG. 9 is a part cross-sectional view on the arrow IX of FIG. 7,

FIG. 10 is a part cross-sectional end view of the mounting of the y-axisgantry of the table of FIG. 1, taken on the arrow X of that Figure,

FIG. 11 is a part cross-sectional view on the arrow XI of FIG. 10,

FIG. 12 is a part cross-sectional end view of the y-axis gantry guide ofthe table of FIG. 1, taken on the arrow XII of that Figure,

FIG. 13 is a schematic block diagram of the table of FIG. I coupled witha small computer and a display tube,

FIG. 14 is a perspective view of a further type of digitising pen foruse with the table of FIGS. 1 to 3,

FIG. 15 is a side view of yet a further type of digitising pen for usewith the table of FIGS. 1 to 3,

FIG. 16 is an end view of the pen of FIG. 15,

FIG. 17 is a perspective view of a further input/output table, and

FIG. 18 is a perspective view of the table of FIG. 17 in a differentposition.

DESCRIPTION OF PREFERRED EMBODIMENTS In this embodiment of theinvention, a dual purpose input/output table indicated generally at 20in FIG. 13 is connected on-line" to a small computer 21 by means of aninterface 22.

The table 20 is shown in more detail in FIGS. 1 to 12 and comprises anupper digitising table surface indicated at 30 which is preferably,though not essentially, transparent. Immediately below the surface 30 abug 31 is mounted for traverse along a gantry arm 32 in a directionacross the table, which in subsequent discussion will be designated they axis; and the gantry arm 32 is itself mounted for longitudinaltraverse along the table, in a direction which in subsequent discussionwill be designated the x axis. The bug 31 is provided with a pen carrierindicated generally at 33 supported below it and arranged to carry aplurality of retractable pens 37 or other marking instruments, which maypreferably be in the form of ballpoint pens.

A further or lower table surface 34 is provided to occupy a planeparallel to that of the table 30 and immediately below the bug 31 sothat as the bug 31 is traversed in the x or y direction the pen 37carried thereon can trace out its path on a sheet of paper or othermaterial laid on the surface of the table 34.

FIGS. 7, 8 and 9 show in more detail the arrangements for the mountingand servo drive of the gantry arm 32. The gantry arm 32 comprises anupper housing which is mounted by means of re-circulating ball bearings81 for traverse along the .r-axis on a rail 82 which is of circularcross-section. The rail 82 is supported at one edge of the table by aseries of posts 83 on a rectangular section tube 84 which is itselfsupported on the lower drawing surface 34. The housing 80 carries a pairof circular cross-section bars 85 which extend across the table in thedirection of the y-axis to a support housing 86 at the far edge of thetable. The bars 85 form a slideway for the bug 31 for its movement inthe y-axis, the bug 31 being supported on the bars 85 which are ofcircular cross-section by means of recirculating ball bearings.

A servo motor 87 with associated reduction gearing 88 is mounted at eachend of the rail 82 for the x-axis drive of the housing 80. Each servomotor 87 is arranged to drive an associated toothed pulley 89 whichengages with a tooth belt 90 extending in a loop from one side of thehousing 80 over the two pulleys and back to the other side of thehousing 80.

On the left hand side of the table as seen in FIG. 1 a digital positionencoder 91 for the x-axis is mounted opposite the servo motor at thatend, and is provided with a grooved pulley 92. A fine close tolerancesteel wire 93 is wound several turns around the pulley 92; and is loopedover a suitable pulley at the opposite end of the rail 82 with the endsof the wire being attached to respective sides of the housing 80. Thusit can be seen that movement of the housing 80 along the x-axis causesproportional rotation of the pulley 92 and thus drives the encoder toproduce an output signal representative of the position of the bug 31 inthe x-axis.

The support housing 86, as best seen in FIG. 12, is provided with a setof three rollers 100, 101 and 102 which are arranged to engage with andrun along a fixed rail 103 to guide the lower end of the gantry arm 32.A further bar 104 extends between the housing 80 and the support housing86 at an angle to the bars 85 to afford rigidity of the gantry arm 32 inthe x-axis direction.

A single servo motor 105 with associated gearing 106 is mounted in thehousing 80 to drive the bug 31 along the gantry arm 32 in the y-axisdirection. In the embodiment shown the servo motor 105 drives a toothedpulley 107 and a tooth belt 108 is looped around that pulley and apulley 109 carried in the support housing 86 with the ends of the beltbeing attached to respective sides of the bug 31. As best seen in FIG.10, a position encoder 110 is mounted with its axis vertical on thehousing 80 with an associated grooved puley 111. A close tolerance steelwire 112 is wound around the pulley 111 looped over a pulley 113 on thesupport housing 86 with the ends of the wire being attached torespective sides of the bug 31. Thus it can be seen that the bug 31 isdriven in the y-axis direction and its position in that axis convertedto an electrical signal in a similar manner to that describedfor thex-axis. A counterweight is provided for the bug 31, the counterweightsliding on a bar 121 between the housing 80 and the support housing 86and being coupled to the bug 31 by means of a wire 122 which is loopedover suitable pulley 123 on the housing 80 and support housing 86.

In one preferred arrangement, the toothed belt 108 is replaced by asteel wire and the pulleys 107 and 109 are replaced by suitable matchingpulleys. This enables the use of smaller pulleys making for a morecompact assembly, while maintaining a smooth drive from the servo motorto the bug 31.

In order to make electrical connection to the housing 80 a cable guideis provided in the form of a bar 130 of circular cross-section extendingparallel to the rail 82 and cables are coiled around that bar asindicated generally at 131 in FIG. 3, and attached to an arm 132 carriedon the housing 80. Cables between the housing 80 and the bug 31 maysimilarly be coiled around the bar 104 to be attached to an arm 133 onthe bug 31.

The encoders may be of optical grating types, synchro-resolvers or ofany type by which the physical position of the bug may be resolved intoelectrical signals, and the representation may be in decimal or binarycode digital form, but is preferably in binary code ready forassimilation by the computer. Again, the encoders may be of anintegrating type giving a continually counted summation of incrementalmovements of the bug in either direction along the x and y axes, or maypreferably be of an absolute position indicating type giving an absoluteread-out of the position of the bug irrespective of initial setting.

It will of course be appreciated that the belt or wire and pulley systemfor moving the bug along the x and y axes may be replaced, in knownmanner, by any suitable means for achieving the traverse of the bug andgantry, such as lead screws or rack and pinion mechanisms.

The output from the digital encoders is also used to drive a visibledigital read-out 24 mounted on the table so that the user may see the xand y co-ordinates of the bug 31 at any time.

Also provided with the table is a "digitising pen" indicated generallyat 40 and which, in one form shown in FIG. 6, comprises a transparentwindow having a pair of cross-wires 41 marked thereon in a holder 42. Anelectric coil 43 is wound in the holder 42 around the cross wires 41 tobe accurately centered therewith. The coil 43 is provided, by way ofawandering lead 44, with a 400 01s alternating electric current from asource within the power supplies (not shown) of the table. Thearrangement is such that the pen 40 may thus be placed at any givenpoint indicated by the cross-wires 41 on the surface of the table 30,and at that point the coil 43 establishes an alternatingelectromagnetic. field beneath the table 30.

The bug 31 is provided with two sensing coils 50 and 51 alignedrespectively with the x and y axes to detect the magnetic field of thecoil 43. The electrical circuitry of the servo-motors of the bug 31 isso arranged that the coils 50 and 51 provide an error signal in respectof each axis when the bug 31 is not accurately centered beneath thecross-wires 41 of the pen 40, and the error signals are used, aftersuitable amplification, to drive the servo motors to correct that errorand thus position the bug beneath the cross-wires 41. A third coil 52 isprovided on the bug 31 by which the bug 31 may be locked onto the pen 40when the bug is correctly positioned beneath the pen.

The servo motor amplifiers are provided with suitable damping meanswhereby the two servo systems are stable and it can thus be seen thatthe system provides for the bug to follow the pen 40 to any positionwithin its scope of movement beneath the table 30 in a continuousmanner. During such movement and when the bug is stationary beneath thepen 40, the digital encoders on the x and y axis servo motors give adigital readout of the position of the bug and thus of the pen 40 on thetable 30. The pen 40 is provided with a push button 45 which isconnected, by way of the wandering lead 44 to the digital read-outcircuits so that on pushing the button 45, the instantaneous read-outposition of the pen 40 taken from the x and y axis digital encoders maybe transmitted to the computer 21.

Thus it can be seen that by placing the cross-wires 41 upon anyco-ordinate point on the table 30, allowing the bug 31 to center itselfbeneath the cross-wires, and then pushing the button 45, theco-ordinates of that point may be transmitted to the computer.

The pen 40 may take other forms in which the crosswires 41 are replacedeither by a more precise pointer or probe, or even by a pen such as aballpoint pen, so that the pen 40 may also be used to draw a visualrepresentation on a sheet of paper or other material carried on thetable 30 at the same time as the digital coordinate positions of variouspoints on the representation are being transmitted to the computer.

The servo motors driving the bug on the x and y axis together with theirrespective position encoders are also arranged so that on transmissionof a changing coordinate position from the computer 21 to the table 20,the bug 31 may be continuously driven to assume a position correspondingto that co-ordinate information, carrying with it the pen 37 to make acorresponding representation on a sheet carried on the table 34.

The pen carrier 33 is mounted on the side of the bug 31 to be indexableabout a vertical axis to bring a selected arc of the pens to anoperative position beside the bug. The carrier is mounted on a bracketwhich is pivotally mounted on the bug 31 about a horizontal axis toallow for lifting of the pen carrier to service the pens. The carrier33, as best seen in FIG. 4, comprises an indexable disc 141 whichcarries six pens 37. Each of the pens is spring loaded to a retractedposition by means of a coil spring 142. The pens 37 shown in FIG. 4 areof a capillary tube nib ink type, but as discussed previously they maybe of any other suitable type; and it is intended that each pen shouldbe filled with a different coloured ink. Indexing of the carrier 33brings each pen successively to an operative position indicated at 143.A pair of solenoids 144 are mounted on the bracket 140 at the position143 and on energisation operate on a lever 145 which is pivotable toengage the top of a pen 37 at that position and drive it down againstthe associated spring 142 to operate on the lower table surface.

As best seen in H0. 5, the carrier 33 is provided with a toothed portion146. A solenoid 147 is provided to move a ratchet pawl 148 in thedirection of arrow 149, on energisation by means of a push rod 150. Theratchet pawl 148 is in engagement with the toothed portion 146 and thusoperation of the solenoid achieves incremental movement of the carrier33. A locking pawl 15] is provided, resiliently biassed into engagementwith the toothed portion 146 to hold the carrier in a given indexposition. The locking pawl 151 is released by the solenoid push rod 149before the pawl 148 is engaged, lost motion being provided between thepawl 148 and a shoulder on the push rod to that purpose.

The solenoids 144 and 147 are energised under the control of thecomputer in use so that the desired pen 37 is brought into operation atany given time in operation of the table to draw out from the computer,the appropriate number of pulses being supplied to the solenoid 147 toindex the carrier 33 and the solenoids 144 being energised to bring theselected pen 37 into use.

Thus it can be seen that the table 20 provides both the input and outputfacility for graphical representations to be transmitted to and takenfrom the computer 21 in digital form. The shaft encoders of table 20 areconnected to the computer 21 by means of an interface system indicatedat 22 in FIG. 13, which in this embodiment is an internationally agreedinterface system based on the United Kingdom Atomic Energy AuthorityHarwell 7,000 Series and referred to by the code name CAMAC. Theinterface system 22 is provided to achieve compatability electricallyand logically between the systems of the table 20 and the computer 21.

The computer 21, which in this embodiment is a Digital EquipmentCorporation Serial PDPS/E, is provided with an input tape facility forthe operating programme, and an output tape facility upon whichdigitised information is stored on a long term basis, and on whichdigitised information may be removed from the computer for useoff-line," if this last facility is desired.

A portion of the surface of the digitising table surface 30 is set asidefor a menu card." The area of the menu card is divided up into smallrectangular blocks to which are assigned individually various items ofinstruction for the computer. Thus if the digitising pen is placed on aparticular menu card rectangle and the push button 45 operated thecomputer is directed to that part of its programme associated withcarrying out the instruction ascribed to that rectangle of the menucard. The computer is programmed to distinguish between the variousinstructions on the basis of the coordinate position of the pen beingwithin the corners of the prescribed rectangle associated with thatinstruction.

The various standard drawing features such as straight lines, circles,arcs or other curves are made the subject of menu card items so thatthey may be digitised by digitising their end points, centre and a pointon the radius as appropriate and entering the appropriate instructionfrom the menu card.

In order that the menu card may be readily changed to provide differentsets of instruction items, for instance when changing from one branch oftechnology to another, the menu card is preferably in the form of anoverlay for the appropriate portion of the table 30, and is preferablyassociated with an individual programme tape for the input tape facilityof the computer. [n this way the difi'erent programme routines necessaryto carry out different instruction items may be readily associated withnot only the menu card, but the co-ordinate position within the menucard. Further programme facilities can be incorporated so that the menucard can be temporarily shifted to an area of the table near an itembeing digitised.

in a typical sequence of operation, the digitising pen may be utilisedto digitise and feed into the computer the details of a straight linebetween two points. It is arranged that to do this the digitising pen isfirst placed on the appropriate area of the menu card corresponding tothe instruction draw a straight line (between two points)," and thatinstruction is then entered in the computer by operation of the button45. The pen is then placed successively on the two points and theircoordinates are entered into the computer. The part of the programme forthe computer associated with that instruction, which is located on theprogramme tape under the control of the computer, is devised to ensurethe realisation that the two following co-ordinate positions are theends of the line and that the line is required between them. Thedigitised information together with the appropriate output drawinginstructions are transferred to the core store of the computer to buildup there into blocks. When a point is reached that the accumulatedinformation in the core store fills a prescribed sized block of corestore capacity, it is arranged that the block of information istransferred under the programme to the output tape for long termstorage.

At any chosen time the "draw out" of digitised information withinthecomputer may be initiated. In order to achieve this, the computer isprovided with an output programme which may be carried on the same tapeas the menu card instruction programme, and which has appropriatesections associated with the output requirements of each menu card itemand arranged to present the co-ordinate information which has beendigitised at the output of the computer in a form suitable for the tablebug to follow. This co-ordinate information output which is in the formof demand coordinates for the bug position at any instant of time, iscompared with the co-ordinates of the bug 31 as indicated by theencoding devices on the x and y axis and the result of this comparisonis used as an error signal to drive the x and y servo motors to move thebug to the position demanded by the computer at any instantaneous time.The output programme for the computer is provided with appropriatesub-routines to interpolate the necessary output co-ordinates in 0.1 mm.increments to produce a continuous line drawn feature in accordance withthe chosen instruction item. The appropriate operation of theretractable pen is also initiated by the output of the computer so thata drawing which has been digitised may be completely reproduced on asheet of paper or other suitable material placed on the lower table 34.

Thus it can be seen that this embodiment provides an interactiveinput/output table for the computer with which it is possible todigitise information, and on which a visual output may be drawn. Theprogramme of the computer is arranged so that a draw-out of informationmay be obtained whether it is in the core store or in long term storageon the output tape of the computer.

By building up suitable menu card items together with appropriateprogramme routines, it is possible to reduce the work involved inpreparing a drawing. Any item required on a drawing which isstandardised, such as a pipe fitting, a nut or bolt, or such as circuitsymbols in electrical diagrams, or the like, may be made the subjects ofa menu card item so that to digitise such an item it is only necessaryto digitise its required position and enter the appropriate instructionfrom the menu card.

In order to digitise alpha-numeric symbols for reproduction on adrawing, the symbol may be digitised point by point, as if it were partof the drawing, or the symbols may be made the subject of menu carditems. As a further alternative, the digitising pen may be provided witha small keyboard by which such alphanumeric information (or other menucard items) may be keyed into the computer together with the coordinatesof the position of the pen which are entered in the same manner asdiscussed previously.

The menu card also preferably includes erasure routines so thatpreviously entered portions of a drawing may be erased from the computerstores.

As an alternative to the tape input on the computer itself for the inputand output programme sections associated with the menu card items, theinput/output table unit may be provided with a separate tape unit,preferably of a type to receive cassettes of tape bearing the input andoutput programme sections for the menu card items.

Referring again to FIG. 13, a cathode ray tube display 23 may also bedriven from an output of the computer. This display may be of thestorage tube type with the trace being written and re-written for dataalready digitised and in the computer store, so that a visualrepresentation of digitised data is presented and maintained as adrawing or plan is digitised. As an alternative the tube display may beused to show the content of a previously digitised drawing so thaterasures and alterations may be considered and made to the data storedin the computer with the effect of those alterations being visible onthe display tube, or being visible as drawn out on the second table 34under the control of the computer.

In a further arrangement the display 23 may be in the form of a scannedvideo display tube in which the presentation is re-written by each scan.In this arrangement it is envisaged that the video display should beobtained from a recorded video magnetic tape loop with each scan orframe of the video display being written from the stored data in thecomputer. As a further development of this arrangement, the computer isprogrammed so that when a moveable mechanism layout is digitised anddisplayed, each frame of the display may be written on the video tapewith the mechanism in successive positions or configurations that it mayassume, with the result that the mechanism may be shown in motion on thedisplay tube. Again with the use of a colour tube, the display can bemade in colour.

FIG. 14 shows an alternative form of digitising pen which isparticularly useful for sketching or drawing an item or layout on asheet on the digitising surface 30 while at the same time entering theco-ordinate of salient points of the sketch or drawing. This pencomprises a body 56 having two feet 57 which are provided with rotatableballs 58 so that the pen may be easily moved over the table surface 30.At a point to make up a triangular support with the feet 57, a pen 53 isprovided, preferably of a ball point type. The pen is also provided withan electrical coil 54 concentric with the pen S3, and a push button 55to enter data as discussed above with respect to the pen 40. Theprovision of the three point support by the balls 58 and the pen 53ensures that the pen and the coil 54 remain normal to the surface 30 inuse; thus avoiding errors which would arise in the positioning of thebug 31 if this criterion was not met, from misorientation of themagnetic field from the coil 54.

FIGS. 15 and 16 show a further type of digitising pen which isparticularly arranged for high precision digitising. This pen has a body60 arranged to be supported on the table surface 30 by two fixed feet 61and two resiliently mounted feet 62. It is provided with a finelypointed stylus 63 which may preferably be tipped with a sapphire tip tomake and retain a fine point. An electrical coil 64 concentric with thestylus 63 is also mounted on the coil together with a push button 65both of which perform the same functions as the corresponding items onthe pens discussed above. In use the body 60 of this pen is rocked onthe feet 61 which are rounded to that purpose, compressing the feet 62to bring the stylus into contact with the table surface 30. Theresilient feet 62 are adjustable and are pre-set to ensure that thestylus 63 and its coil 64 are accurately normal to the table surface 30when the stylus point is in contact therewith. In order that the usermay clearly observe the positioning of the stylus on the table surface30, the body 60 is cut away in plan view and a magnifying lens 66mounted thereon to afford a view of the stylus point from above and theleft as seen in FIG. 6, and above and behind as seen in FIG. 7.

FIGS. 9 and 10 shown an alternative arrangement of the input/outputtable unit in which the first or digitising table surface 70 and thesecond or draw-out table surface 71 are on opposite faces of a singletable board 72. The board 72 is pivotally mounted at each end on a stand73 and locking means 74 is provided at one end whereby the table board72 may be held either with the surface 70 upwards as shown in FIG. 9 fordigitising, or with the surface 71 upwards as shown in FIG. 10 forreproducing digitised information in a draw-out.

Other features of this table unit arrangement may be similar to thosediscussed above for the previous table unit and are given the samereference numerals.

The table board 72 must be of a material which is non-magnetic so as notto interfere with the influence of the coil of the digitising pen 50upon the sensing coils of the bug 31 which is on the other side of thetable board. It has been found that a paper honeycombe structure withsuitable facing surfaces and inpregnated with synthetic plasticsmaterials will fulfil these requirements whilst also affording thedesirable dimensional stability for the board.

On either arrangement for the table unit, drawing paper may be providedin a continuous roll for drawing out digitised information. A supplyroll (not shown) and a take-up roll (not shown) are mounted on oradjacent the second table surface so that the roll of paper may bepassed over that surface between them. The computer output programme isarranged to index the paper, by means of a suitable servo-system so thatsuc cessive digitised drawings or plans may be drawn out on the rollautomatically. Suitable interlocking arrangements are provided toprevent operation of the bug 31 in the draw-out mode when the paper isindexing, and to cater for when the paper runs out.

In an alternative arrangement of the gantry arm 32, the housing issupported on the rail 82 by means of air bearings to further reduce thefriction and "stiction" of the housing on the rail. The bug 31 may alsobe supported by air hearings on the bars 85, air being supplied to thosebearings by means of suitable flexible piping. Again the three rollersupport of the housing 86 on the rail 103 may be replaced by airbearings. It is considered that air bearings are particularlyadvantageous in the support and guidance of the gantry arm 32 in view ofthe considerable mass thereof, but small improvements are gained fromthe use of air bearings for the bug 31 on the gantry arm.

In an alternative arrangement of the bug and pen, the sensing coils 50and 51 and the third coil 52 may be mounted in the pen on index member40, and the sensing coil 43 may be mounted on the bug 31 to produce anexactly equivalent arrangement to that discussed above.

The first or digitising table surface 30 is hingedly mounted so that itmay be raised to provide access to the mechanism of the bug drive andposition encoders, suitable counter balance means being provided tofacilitate this.

What we claim is l. A digitising table unit, comprising a first ordigitising table surface, freely movable digitising index member whichmay be placed at any chosen position within the area of the first tablesurface, a bug mounted below the first table surface to be movable alongx and y axes by means of servo-motors to a position corresponding to theposition of the digitising pen when the pen is place on the first tablesurface, encoding means arranged so that the x and y coordinates of theposition of the bug and therefore the pen is converted into electricalsignals indicative of that position, a second table surface occupying aplane parallel to that of the first table surface and located in aposition below the area of the first table surface, and retractable penmeans mounted on the bug and engageable with the second table surfacewhereby movement of the bug may trace out a representation on the secondtable surface.

2. A table unit as claimed in claim 1, including an x axis slidewayaligned in said x axis, a gantry arm slidably mounted on said x axisslideway, and a y axis slideway on said gantry arm, said bug beingslidably mounted on said y axis slideway.

3. A table unit as claimed in claim 2, wherein said gantry arm and saidbug are slidable on said slideways by means of re-circulating ballbearings.

4. A table unit as claimed in claim 2, wherein said gantry arm isslideable on said x axis slideway by means of air bearings.

5. A table unit as claimed in claim 2, wherein said x axis and said yaxis servo motors each drive said gantry arm and said bug respectivelyby way of a flexible looped element extending around a pair of pulleysdisposed one at each end of the respective slideway, said loopedelements being attached respectively to the gantry arm and the bug, andone of the pairs of pulleys being driven by the respective servo motor.

6. A table unit as claimed in claim 5, wherein said looped element is atoothed belt.

7. A table unit as claimed in claim 5, wherein said looped element is aflexible wire.

8. A table unit as claimed in claim 2, wherein said encoding meanscomprises an x axis rotary encoder and a y axis rotary encoder, and saidencoders are each driven respectively from said gantry arm and said bugby means of a flexible looped element extending around a further pair ofpulleys disposed one at each end of the respective slideways, saidlooped elements being attached respectively to the gantry arm and thebug and one of the respective pairs of pulleys driving the respectiveencoder.

9. A table unit as claimed in claim 8, wherein said looped element is aclose tolerance steel wire.

10. A table unit as claimed in claim 8, wherein each said looped elementis wound around the respective one of said pairs of pulleys for aplurality of turns.

11. A table unit as claimed in claim 2, wherein said bug is providedwith a moveable counter balance weight.

12. A table unit as claimed in claim 1 comprising an indexable pencarrier mounted on said bug and a plurality of pens in said carrier,said carrier being indexable under the control ofa computer to bring aselected one into operation.

13. A table unit as claimed in claim 1, wherein said index member isprovided with an electric coil arranged to create a magnetic field, andsaid bug is provided with a sensing coil aligned in said 1 axis and asensing coil aligned in said y axis to detect said magnetic field andprovide signals indicative of an error in the position of said bugrelative to said index member for the control of said servo motors.

14. A table unit as claimed in claim 13, including a third electric coildisposed on said bug whereby said bug may be locked magnetically to saidindex member when in correct relative position thereto.

15. A table unit as claimed in claim 1, wherein said bug is providedwith an electric coil arranged to create a magnetic field, and saidindex member is provided with a sensing coil aligned in said x axis anda sensing coil aligned in said y axis to detect said magnetic field andprovide signals indicative of and error in the position of said bugrelative to said index member for the control of said servo motors.

16. A table unit as claimed in claim 15, including a third electric coildisposed on said index member whereby said bug may be lockedmagnetically to said index member when in correct relative positionthereto.

17. A table unit as claimed in claim 1, wherein said index memberincludes a window aperture and a pair of cross wires disposed thereinfor alignment of the member with a selected position on said digitisingtable surface.

18. A table unit as claimed in claim 1, wherein said index memberincludes three point means for supporting it on said digitising surfaceand one of said points comprises a pen for indicating a chosen positionand drawing on said digitising surface.

19. A table unit as claimed in claim 1, wherein said index memberincludes means to support it on said digitising surface and a finelypointed member for indicating a chosen position, and further includesoptical magnifying means for a user to view the area of the digitisingsurface beneath said pointed member.

20. A table unit as claimed in claim 1 wherein said first or digitisingtable surface is provided on a transparent sheet.

21. A table unit as claimed in claim 1, wherein said first and secondtable surfaces are provided on opposite faces of a single board, saidboard being pivotally mounted about a horizontal axis whereby said firstor said second table surface may be presented uppermost.

22. A combination comprising a digital computer, an input digitisingtable and an interface means connecting the table to the computer, thetable comprising a first or digitising table surface, a freely movabledigitising index member which may be placed at any chosen positionwithin the area of the first table surface, a bug mounted below thefirst table surface to be movable along x and y axes by means ofservo-motors to a position corresponding to the position of thedigitising pen when the pen is placed on the first table surface, encod-

1. A digitising table unit, comprising a first or digitising tablesurface, freely movable digitising index member which may be placed atany chosen position within the area of the first table surface, a bugmounted below the first table surface to be movable along x and y axesby means of servo-motors to a position corresponding to the position ofthe digitising pen when the pen is place on the first table surface,encoding means arranged so that the x and y coordinates of the positionof the bug and therefore the pen is converted into electrical signalsindicative of that position, a second table surface occupying a planeparallel to that of the first table surface and located in a positionbelow the area of the first table surface, and retractable pen meansmounted on the bug and engageable with the second table surface wherebymovement of the bug may trace out a representation on the second tablesurface.
 2. A table unit as claimed in claim 1, including an x axisslideway aligned in said x axis, a gantry arm slidably mounted on said xaxis slideway, and a y axis slideway on said gantry arm, said bug beingslidably mounted on said y axis slideway.
 3. A table unit as claimed inclaim 2, wherein said gantry arm and said bug are slidable on saidslideways by means of re-circulating ball bearings.
 4. A table unit asclaimed in claim 2, wherein said gantry arm is slideable on said x axisslideway by means of air bearings.
 5. A table unit as claimed in claim2, wherein said x axis and said y axis servo motors each drive saidgantry arm and said bug respectively by way of a flexible looped elementextending around a pair of pulleys disposed one at each end of therespective slideway, said looped elements being attached respectively tothe gantry arm and the bug, and one of the pairs of pulleys being drivenby the respective servo motor.
 6. A table unit as claimed in claim 5,wherein said looped element is a toothed belt.
 7. A table unit asclaimed in claim 5, wherein said looped element is a flexible wire.
 8. Atable unit as claimed in claim 2, wherein said encoding means comprisesan x axis rotary encoder and a y axis rotary encoder, and said encodersare each driven respectively from said gantry arm and said bug by meansof a flexible looped element extending around a further pair of pulleysdisposed one at each end of the respective slideways, said loopedelements being attached respectively to the gantry arm and the bug andone of the respective pairs of pulleys driving the respective encoder.9. A table unit as claimed in claim 8, wherein said looped element is aclose tolerance steel wire.
 10. A table unit as claimed in claim 8,wherein each said looped element is wound around the respective one ofsaid pairs of pulleys for a plurality of turns.
 11. A table unit asclaimed in claim 2, wherein said bug is provided with a moveable counterbalance weight.
 12. A table unit as claimed in claim 1 comprising anindexable pen carrier mounted on said bug and a plurality of pens insaid carrier, said carrier being indexable under the control of acomputer to bring a selected one into operation.
 13. A table unit asclaimed in claim 1, wherein said index member is provided with anelectric coil arranged to create a magnetic field, and said bug isprovided with a sensing coil aligned in said x axis and a sensing coilaligned in said y axis to detect said magnetic field and provide signalsindicative of an error in the position of said bug relative to saidindex member for the control of said servo Motors.
 14. A table unit asclaimed in claim 13, including a third electric coil disposed on saidbug whereby said bug may be locked magnetically to said index memberwhen in correct relative position thereto.
 15. A table unit as claimedin claim 1, wherein said bug is provided with an electric coil arrangedto create a magnetic field, and said index member is provided with asensing coil aligned in said x axis and a sensing coil aligned in said yaxis to detect said magnetic field and provide signals indicative of anderror in the position of said bug relative to said index member for thecontrol of said servo motors.
 16. A table unit as claimed in claim 15,including a third electric coil disposed on said index member wherebysaid bug may be locked magnetically to said index member when in correctrelative position thereto.
 17. A table unit as claimed in claim 1,wherein said index member includes a window aperture and a pair of crosswires disposed therein for alignment of the member with a selectedposition on said digitising table surface.
 18. A table unit as claimedin claim 1, wherein said index member includes three point means forsupporting it on said digitising surface and one of said pointscomprises a pen for indicating a chosen position and drawing on saiddigitising surface.
 19. A table unit as claimed in claim 1, wherein saidindex member includes means to support it on said digitising surface anda finely pointed member for indicating a chosen position, and furtherincludes optical magnifying means for a user to view the area of thedigitising surface beneath said pointed member.
 20. A table unit asclaimed in claim 1 wherein said first or digitising table surface isprovided on a transparent sheet.
 21. A table unit as claimed in claim 1,wherein said first and second table surfaces are provided on oppositefaces of a single board, said board being pivotally mounted about ahorizontal axis whereby said first or said second table surface may bepresented uppermost.
 22. A combination comprising a digital computer, aninput digitising table and an interface means connecting the table tothe computer, the table comprising a first or digitising table surface,a freely movable digitising index member which may be placed at anychosen position within the area of the first table surface, a bugmounted below the first table surface to be movable along x and y axesby means of servo-motors to a position corresponding to the position ofthe digitising pen when the pen is placed on the first table surface,encoding means connected to the interface, whereby the x and ycoordinates of the position of the bug and therefore the pen on thefirst table surface may be converted to electrical signals indicative ofthat position and transmitted to the computer, a second table surfaceoccupying a plane parallel to that of the first table surface in aposition below the area of the first table surface, and retractable penmeans mounted on the bug and engageable with the second table surfacewhereby movement of the bug may trace out a representation of the secondtable surface.
 23. A combination according to claim 22, wherein acathode ray tube is connected to said computer for visual display ofdigitized information in said computer.